Like the portly kid who’s chosen last for dodgeball in gym class, online ad network ValueClick (VCLK) must be feeling pretty left out.

Now that Microsoft (MSFT) is offering to pay $6 billion for ValueClick rival aQuantive (AQNT), a stunning 85 percent premium, ValueClick is the last of the major publicly traded online marketing companies that hasn’t been scooped up by a bigger fish in the Internet advertising pond. 24/7 Real Media (TFSM) agreed to be taken over by ad agency WPP Group (WPPGY) on Thursday. 

But investors appear to be betting that ValueClick might be the next online ad firm to get taken over. The stock was up more than 10 percent in pre-market trading Friday. And shares are already up 18 percent year-to-date.

Still, it’s getting tougher to imagine who might be interested in buying ValueClick since Microsoft’s rivals have already made significant deals in the past month to bulk up in the online ad network arena.

Google (GOOG) started off this merger frenzy in early April when it announced it was buying privately held DoubleClick for $3.1 billion. And Yahoo! (YHOO) followed suit later that month with a deal to purchase the remaining 80 percent in Right Media that it didn’t already own, for $680 million.Sure, maybe a company like Barry Diller’s IAC (IACI), which owns Ask.com, might decide that it now needs to become a player in the online ad network or online ad agency business. 

Or maybe another traditional ad agency such as Omnicom (OMC) or Interpublic (IPG) might want to bolster their digital business like WPP has just done. In fact, another big Madison Avenue firm, Publicis (PUB) has also made a notable deal in the online marketing area, agreeing to buy interactive ad agency Digitas late last year.  

But as I pointed out earlier this month, there are also several privately held online marketing firms, companies like Tribal Fusion and Burst Media, that could also be attractive, and would probably cost a lot less than ValueClick. Another hot privately held online ad company, Gorilla Nation, announced Friday that it was receiving more than $50 million in financing from private equity firm Great Hill Partners.

So even though ValueClick may now face more of an urgent need to sell as all of its rivals pair up with much larger, deep-pocketed parents, the company - and its investors - may have to get used to sitting on the sidelines.